1. Field of the Invention
The present invention generally relates to optical information processing, and more specifically to information processing device using an optical shutter elements, and an optical logic device using an optical shutter elements.
2. Description of Background Information
In the field of optical information processing, analog type optical parallel processing originated from the invention of holography has reached the stage of practical application. The analog type optical parallel processing is applied mainly for picture information processing in various factory production lines. In such applications, there however is a serious limitation that a different and rather complex optical system is required separately for each of the objects of the application. On the other hand, combination of image input using a solid image pickup element and image processing by means of a computer has made rapid progress owing to the advancement of the device technology. Because of these reasons, the analog type optical parallel processing is gradually losing its role. Moreover, in view of the limitation in the accuracy of the analog processing, and for effectively utilizing the knowledge of electronics which has been accumulated for a long period, digital type optical information processing is now acquiring much interest in this field.
On the other hand, a major advantage inherent in the optical information processing and which has been pursued therewith is its fast speed characteristic as compared with conventional electronic circuits. Besides, the elimination of electro-magnetically induced noises can be expected by the use of optics.
Research of the digital type optical information processing is taking place on the basis of the above mentioned fundamentals. The optical information processing is divided into two sorts of techniques, namely, time series processing and optical parallel processing. Specifically, the optical digital time series processing has been developed, aiming at the replacement of switching elements typically represented by transistors in electronic circuits of conventional integrated circuits with optical shutters having ultra high speed, thus replacing electrons with photons as a driving medium. Furthermore, the development of the time series processing has a target to take over the architecture of conventional electronic circuits as it is, and to realize very high speed operations. As an example of the optical shutter used for this purpose, a crystal having non-linear type photo responsive refractive index was applied to the Fabry-Perot interferometer by E. Abraham et al. (E. Abraham et al., Scientific American, Feb. 63 (1983)). In this application, a switching speed having the order of 10.sup.-12 S has been attained.
On the other hand, the optical parallel processing is targeted to accomplish a high operation speed by means of a high speed processing of multiple input type optical information such as image input or by the parallel processing of operations executed in usual computers. In this case, an operation speed value obtained by dividing the speed of optical shutter by the number of parallel processing corresponds to the speed of optical shutters in the above mentioned time series processing. This means that the ultra high speed optical shutter is not necessarily required, and the integration density and the easiness in constructing a logical operation device become very important factors. Especially, in the processing of image input, the high density integration of the optical shutter becomes important because the number of light receiving parts and optical shutters determines the resolution of image because it corresponds to the number of picture elements in usual display panel, and determines the resolution of the picture. Therefore, the integration scale of elements such as optical shutters may be a more important factor.
As an example of application of the parallel processing, a type of digital parallel processing is proposed by Seko (A. Seko, Applied Physics 53, 409 (1984)). The purpose of this application by Seko is to evaluate a pattern matching between an input image and a reference image. To this end, an exclusive OR signal of both images is generated and outputted as a light output. The structure used for generating the exclusive OR signal is as follows: at first each of the input image and the reference image is separated into two of the same images by a light branching operation. One of the thus obtained two images is inverted respectively to provide an inverted image of the input image and an inverted image of the reference image. These inverted images of the input image and the reference image are then combined with the remaining reference image and the input image produced by the light branching operation, so as to provide logical products. The above mentioned optical exclusive OR signal is obtained by combining these logical products, to produce a logical sum. Each stage of the inversion, generation of a logical product, and generation of a logical sum is performed by a two-dimensional optical information processing device in which optical logic elements having a respective one of these functions are arranged in the form of a two-dimensional array. To provide a total system, a plurality of the two-dimensional optical information processing devices are arranged in series manner.
The two-dimensional optical information processing device is, for example, presented by an integration of optical fibers (A. Seko et al., Applied Optics, 18, 2052 (1979)). However, in view of its readiness for integration and form production, devices using liquid crystal are considered most promising. As an example, a two-dimensional optical, information processing device in which a photo conductive is introduced in a liquid crystal panel is proposed (R. A. Athale et al. Optical Eng., 18, 513 (1979)).
FIG. 15 shows the structure of optical logic elements in the prior art optical information processing device by Athale et al. In actual arrangement, these elements are arranged in two-dimensional manner, to form a two-dimensional array, and the liquid crystal layer is provided in combined form and formed in succession.
The structure of this optical logic device is as follows.
The device comprises clear electrodes 1510, 1520, and 1530, a photo conductive layer 1540, and a transparent layer 1550, and a liquid crystal 1560. In order to obtain a desirable operation of the device, a polarizing plate is suitably combined with the liquid crystal so that the liquid crystal transmits a light incident there upon when a sufficient voltage is applied, and it does not transmit the light when the applied voltage is lower than a threshold level. The operation of the device is as follows. Under application of the above mentioned voltage whose level is higher than the threshold level across the clear electrodes 1510 and 1530, when an incident light 1570 is not present, the photo conductive layer 1540 exhibits a high resistance state, and for the most part of the voltage applied across the clear electrodes 1510 and 1530 is absorbed by a voltage drop at the photo conductive layer so that almost no voltage is applied to the liquid crystal layer 1560. Therefore, even if there is another incident light 1580, a level of an output light 1590 will be equal to zero. On the other hand, when the incident light 1570 is present, the photo conductive layer 1540 becomes conductive, to control the potential of the clear electrode 1520 to be equal to the potential of the clear electrode 1510. In this state, the liquid crystal layer 1560 is applied with a voltage higher than the threshold level and the output light 1590 will be generated when another incident light is supplied. In addition, the above explained operation of the device can be reversed by changing the arrangement of the polarizing plate, so that the output light will not be generated when the incident light 1570 is present.
The above described operation is summarized such that the output light 1590 is generated only when both of incident lights are present. This means that a logical product (AND) of two incident lights is obtained. Furthermore, by changing the arrangement of the polarizing plate, other logical operations such as an inversion (NOT) can be obtained.
However, with this prior art optical logic device, there are several limitative conditions which make it difficult to provide a two-dimensional optical information processing system having enough freedom of design and is sufficiently reliable.
The problems with the conventional device are as follows.
1. It is general to arrange several two-dimensional optical information processing elements in a serial manner such as in the structure used in the pattern matching of Seko. With this type of structure, light outputs from different layers are attenuated differently depending on the number of layers each light has passed. Therefore, the level of the output light will have various levels, and causing an increase in the possibility of malfunctions.
2. It is not possible to provide an operation for generating a logical product among more than two inputs.
3. Logical sum (OR) operation can be attained if only transparent layers are used in the device, i.e., without the use of the photo conductive layer. However, in such a case, output signals will have several different levels, and it is very likely to cause malfunctions as in the above mentioned problem 1.
4. Logical inversion can be attained, as mentioned before, by arranging the polarizing plate suitably. However, in practice, in a structure including a plurality of optical logic devices shown in FIG. 15 are arranged in a single liquid crystal panel, it becomes necessary to put polarizing plates on the liquid crystal corresponding to each logical element in a mosaic form in order to provide functions of logical product and logical inversion in the same structure. This is a quite difficult task in producing a high density optical logic element.
5. In a multilayer structure arrangement mentioned in connection with the above point 1, a portion of the device in which the photo conductive layer is disposed is not transparent. This means that the transparent portion of the device decreases as the number of layers increases, and this is an obstacle to the production of a highly integrated structure.